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[AMPS] 3dB Pads

To: <amps@contesting.com>
Subject: [AMPS] 3dB Pads
From: km1h@juno.com (km1h @ juno.com)
Date: Fri, 10 Oct 1997 09:54:52 EDT
On Thu, 09 Oct 97 19:28:44 PDT "james fisher" <k4gf@worldnet.att.net>
writes:
>Oct, 1997, from K4GF:  My winter project this year will be to
>up-date and rebuild an old amp using a pair of 811-A tubes.  Of
>several ways to limit the available 100 watts of drive I would
>like to simply install an attenuator of 3 to 4 db in the input.
>
>Hamcalc will tell me the values of resistance, but I am concerned
>about the dissipation and any special features of the resistors.
>
>I am guessing now, but the peak dissipation would be some 60
>watts or so and the average one-fifth of that.  Anyone who can
>help would save me lots of R & D time.  Thanks...Jim Fisher

Pi and T pads values are also in every ARRL Handbook.

I have built 3dB Pi pads with three 2 W carbon resistors that work well
with 50W of SSB input. I build them in a cast aluminum box and use
thermal grease to couple them to the metal and a small finned heat sink
is bolted externally.  After several years of use the R values have not
changed. You might try experimenting with a similar design using 2
resistors for each leg.  One problem with paralleling multiple resistors
is the lead inductance might affect 10M input VSWR.

An alternative is to replace the 811A's with 572B's. A pair of those can
easily handle 100W. 

GL   Carl   KM1H

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